1. Field of the Invention
The present invention relates to a valve comprising an automatic bleed device. It has an application in restricting the pressure within an enclosure containing an incompressible fluid, in particular when said enclosure is pressurized by means of a volumetric pump.
2. Description of the Prior Art
Safety valves of the so-called "relief or over-pressure type" are known. Such valves are provided with a ball adapted to abturate an orifice, or port, said ball being maintained on its seat by means of a spring-biassed piston. The force exerted by said spring can be controlled through a screw for adjusting its compression. It is thus possible to adjust the minimum pressure above which the ball is to be raised from its seat. Such a valve is mounted on the wall of an enclosure containing a pressurized fluid. Whenever the pressure within said enclosure rises beyond a given amount, the ball rises and the fluid contained in said enclosure flows outedly therefrom. In this manner any risks of disturbance or failure are avoided such as might occur in case of overpressure in the enclosure in question.
Not unfrequently, however gas bubbles (e.g. air-bubbles) are to be found in the fluid contained in said enclosure; that is why on the enclosure containing the pressurized fluid, an automatic bleed device is usually mounted in addition to the conventional safety valve. The function of the latter is due to vent from the enclosure a certain amount of fluid, e.g. oil, adapted to drive therewith the gas contained in said enclosure. Quite obviously the selection of the position of said automatic bleed device is important. Said device, indeed, should be installed in the topmost portion of the enclosure, where the gas contained in the fluid tends to gather.
Such an automatic bleed device can comprise a body fixed to the enclosure wall, e.g., by means of screws, said body containing an inlet nozzle obturated by means of a needle applied against said nozzle by means of a piston driven through a balancing spring. The pressure exerted by said balancing spring on said nozzle-inlet-obturating needle can be controlled by means of an adjusting screw. It is thus possible to adjust the force exerted by such a spring in such a manner that, through the pumping pulses, said needles thus balanced by the spring-driven balancing piston be imparted an alternating movement and allow the gas-fluid emulsion to flow through the annular clearance between said needle and said body of the automatic bleed device.
In the long run, however, the gas tends to accumulate at the safety-valve inlet, thus forming a compressible cushion, which is a nuisance, especially if the installation comprises a volumetric pump, e.g. a piston or a diaphragm-pump, the volumetric efficiency is decreased by such a cushion. Said compressible-cushion-forming gas can be vented from the enclosure only provided the hydraulic nominal pressure is higher than the safety-valve calibration-pressure, which in fact rarely occurs. This is the reason why the safety valve should be calibrated by hand whenever said gas-cushion becomes too thick thus causing the pump flow-rate to decrease to an overly large extent.
A further drawback of such a device lies in the fact that it requires the installation of two separate components and of two ducts for insuring the return of oil into the tank or cistern, from the inside of which the fluid is sucked by the pump, thus increasing the cost-price and bulkiness of said device.